A Correlative Method for Structure of Solid Saturated Hydrocarbons

Abstract

A number of the correlative methods for determining structure of liquid hydrocarbons are unsuitable for the solid saturated hydrocarbons present in petroleum waxes. In most cases, these methods employ correlation of physical property measurements made at 20 C, but suitable extrapolations are not available for use at higher temperatures. Furthermore, most of these require accurate measurements of molecular weight, and with solid hydrocarbons in the molecular weight range above 400 these have been difficult to obtain with sufficient accuracy (5, 6). In a recent study of the composition of microcrystalline waxes, it was desirable to have correlative methods which could employ relatively simple measurements at elevated temperatures to obtain an estimation of the approximate composition of isolated fractions and, if possible, learn something of the architecture and symmetry of the components. Recently Schiessler (1) has published criteria defining the limiting molecular structure of hydrocarbons which can be reacted with urea and thiourea to form clathrate compounds. Thus a given sample or fraction can be preliminarily classified. A scheme for this is shown in Fig. 1. Since the adducts of urea and thiourea can be easily decomposed to yield the reacting hydrocarbons, they can be further fractionated and examined to provide additional information. The procedure for removing aromatics from hydrocarbon mixtures by adsorbtion is well known. The method employed in this laboratory consists of dissolving each 100 g of sample in 1000 ml of isooctane and percolating through a 1-in. diameter column, 6 ft. long, packed with 700 g of 100 to 200 sieve Davison silica gel. This solution is followed with 700 ml of pure isooctane to displace the unadsorbed material. The filtrate is distilled to remove the isooctane, followed by steam and vacuum stripping. The adsorbed aromatics may be eluted by passing benzol or a benzol and alcohol mixture through the column.